Disposal of ash from coal preparation plant tailings

ABSTRACT

The ash in coal preparation plant tailings is converted into inert water-insoluble pellets by continously delivering the tailings in the form of a slurry of water and particles of combustible material and ash to a combustion zone in which a fluidized bed of those particles is maintained. Self-sustaining combustion of the combustible material in the bed is established and maintained at a temperature that causes the ash particles to agglomerate to form said pellets, which are removed from the bed at a rate that maintains the desired pressure drop across the bed.

United States Patent 1 Notary et a1.

Apr. 23, 1974 1 DISPOSAL OF ASH FROM COAL PREPARATION PLANT TAILINGSInventors: Joseph A. Notary, Monroeville;

Donald E. Metheny, Pittsburgh, both of Pa.

Heyl & Patterson, Inc., Pittsburgh, Pa.

Filed: May 22, 1973 Appl. No.: 362,808

[73] Assignee:

US. Cl /7 S, 110/28 .1, 110/165 R Int. Cl. F23d 19/00 Field of SearchllO/7 S, 8 R, 28 J, R,

[ 56] References Cited UNITED STATES PATENTS 3/1964 Sage et al. 110/75/1967 Albertson et al. 110/8 8/1968 Tada 110/8 3/1969 Godel 110/28Primary Examiner-Kenneth W. Sprague Attorney, Agent, or Firm-Brown,Murray, Flick &

Peckham [57] ABSTRACT 9 Claims, 1 Drawing Figure 400/ 7/0/1441 COALF/NES 14 4 75R 35 WPUIQ DUST COLLECTOR T/I/CA/ENER 4 peas 5475PATENTEUAPR 23 191+ ENSQ ww wkm3wq$ ww v I mNm v QWEQSQ m ll DISPOSAL OFASH FROM COAL PREPARATION PLANT TAILINGS The waste or tailings from coalpreparation plants is in the form of a slurry of water and solidparticles. The solids in the slurry are made up of coal particles, ashparticles and particles of ash and coal combined. The ash is any solidmaterial that has no BTU values and is mostly clay. Getting rid of suchcoal plant tailings safely without polluting the environment is aserious problem. In the past, the tailings have often been simply runout onto the land, where rain has washed them into streams and riversthat they clogged with silt. More recently, coal preparation planttailings have been confined in lagoons or dammed up streams, but it hasbeen found that this is hazardous because the dams and the lagoon wallssometimes break and suddenly release their contents and cause flooding.If the ash in the tailings could be disposed of satisfactorily in amanner that would not pollute the environment, getting rid of thecombustible material and the water would be relatively easy. The ash hasbeen the problem..

It is among the objects of this invention to provide a method ofdisposing of the ash from coal preparation plant tailings, which issimple, inexpensive, continuous and non-polluting of the environment,and which simultaneously disposes of the water and combustible material.

Apparatus for carrying out this process is illustrated diagrammaticallyin the accompanying drawing.

Referring to the drawing, coal preparation plant tailings, consisting ofa slurry of water and particles of combustible material and combinedcombustibles and ash, are conducted from the plant through a conduit 1to a large collector or thickener 2 provided with an outlet in itsbottom. This outlet is connected by a pipe 3 to a constant speed pump 4that pumps the slurry through a pipe 5 to a control sump 6. The densityof the slurry can be measured by a conventional instrument 7 and if thedensity falls below a desired value some of the slurry can be returnedto the thickener through a pipe 8 leading from the control sump. Aslurry with a solids content between 35 percent and 45 percent by weighthas been found to be best as will be explained later, the solids make-upbeing in the neighborhood of about 50 percent coal and 50 percent ash byweight.

The bottom of the sump has an outlet connected by a pipe 10 to avariable speed pump 11 that delivers the slurry through a pipe 12 intothe side of a furnace 13, which serves as an agglomerator as will beexplained presently. In the lower part of the furnace there is a grate14 provided with a multiplicity of very small openings through it.Projecting into the furnace beneath the grate is a start-up burner 15that may burn oil or gas. Also connected with the furnace beneath thegrate is a pipe 16, to which air is supplied under pressure. This airflows upwardly through the openings in the grate in order to form afluidized bed of particulate material on the grate.

Before this apparatus is put in operation, a bed of inert granularmaterial, such as sand, is spread on the grate. Then the burner 15 islit and air is delivered to the furnace below the grate through pipe 16.The air flow is sufficient to cause the sand to form a fluidized bedabove the grate. The burner continues to operate until the temperatureof the sand is high enough to ignite coal particles. At that time thepumps are started in operation in order to feed the slurry into thefurnace above the grate. The solid particles in the slurry mix with thesand and become a part of the fluidized bed. As soon as the combustiblematerial in the bed ignites and starts to burn, the burner is turnedoff. After that the combustible material in the bed is depended upon tomaintain combustion in the furnace, thereby reducing the cost ofoperation. in other words, burning ofthe fluidized bed isself-sustaining. The high temperature in the furnace causes the slurrywater that is delivered to it to immediately vaporize and flow up andout through the stack 18 connecting the top of the furnace to the sideof a dust collector 19. The top of the dust collector has an outlet forthe water vapor and combustion gases, which can pass into the atmospherewithout contaminating it.

It is a feature of this invention that the furnace is maintained at sucha temperature that the ash particles in the fluidized bed, includingthose from which coal fines are burned, agglomerate by forming hardpellets that may be any size up to about a quarter of an inch across.Why this agglomeration takes place is not fully understood, althoughsurface fusion of the particles seem to play a part, but the pelletsthat are thereby formed not only are inert but they also are insolublein water. Furthermore, they will not break up into fines again and washaway when rained upon. They can be used for landfill or dumped into oldmines. They even can be used as aggregate in the making of buildingblocks and the like. There are many possible uses for the pellets.

One way of controlling the temperature in the furnace when the solidscontent of the slurry fed into it is not below about 35 percent byweight is to adjust the speed of pump 1 1 to increase or decrease thefeed. Another way is to operate the pump at constant speed and divertsome of the slurry back to control sump 6 if a reduction in feed isdesired in order to reduce the temperature in the furnace. On the otherhand, if the solids content of the slurry is below about 35 percent byweight, burning in the furnace will not be selfsustaining without somehelp. This help can be furnished by heating the fluidizing air in a heatexchanger or preheater 21. The hotter this air entering the furnace, thehigher the temperature in the furnace. The temperature of the preheater,when used, can be controlled by a conventional temperature measuringinstrument 22 inserted in the upper end of the furnace or in the stackand operatively connected with the preheater in any well-known manner.Preheating is not required when the solids content is above about 35percent by weight, because in that case the material will furnish enoughBTU s to sustain combustion. If the solids content of the slurrydelivered to the furnace is above about 45 percent by weight, too muchheat is produced and the feed must be cut back. This means that theequipment then will not be operating at its optimum capacity.

It has been found that for best results the furnace temperature shouldbe between about l,400 and 1,700 F. In this range the ash particles willform pelletsand the bed will remain fluidized. A minimum temperature ofabout l,400 F is required in order to sustain combustion in the furnace.On the other hand, if the temperature starts to rise above about l,700F, the character of the fluidized bed changes as the ash starts to fusetogether into large lumps and the bed becomes defluidized.

For removing the pellets from the agglomerator, an inclined pipe 24extends from just above the grate down through it and out the side ofthe agglomerator to a conveyor 25 or other means for carrying thepellets away. The pellets in the fluidized bed will flow into the upperend of this pipe and slide down through it and onto the conveyor. Sincethe pressure drop across the fluidized bed should be maintained asconstant as possible for proper operation, the outlet pipe for thepellets is provided with a throttling valve 26 to control the flow ofpellets away from the bed at a rate that will maintain the desiredpressure drop across the bed. This valve is operated in accordance withthe pressure drop of the fluidizing air across the bed. If, for example,the pressure drop starts to fall, the change, through the agency ofpressure-sensitive control device 27 connected with the inside of thefurnace by tubes 28 and 29, will cause the throttling valve to operateto reduce the size of the passage through it in order to slow down theescape of pellets from the fluidized bed.

Dust deposited in the dust collector can be removed through a pipe 31and a three-way valve 32 that delivers the dust either to the pelletremoval pipe 24, whereby the dust mixes with the pellets and is carriedaway, or back to the furnace. if it happens that the tailings includesuch a small proportion of coal particles that it is difficult tomaintain combustion in the agglomerator, additional fine particles ofcoal can be added to the slurry in sump 6 through a three-way valve 35and a pipe 36. Or, by turning the valve, the coal fines can be deliveredfrom the valve to another pipe 37 that will supply them to the fluidizedbed in dry condition.

If the fusion point of the ash in a given collection of tailings is sohigh that the ash particles do not agglomerate in the furnace, theirfusion point can be lowered by adding to the slurry in sump 6 a materialthat will lower the fusion point of the ash. This material can besupplied through a pipe 39 from a hopper 40. Soda ash is suitable forthis purpose, but other low melting point inorganic compounds canlikewise be used.

It will be seen that the process disclosed herein provides a continuousmethod of converting the ash in coal preparation plant tailings into aninert product that not only is non-polluting but also can be useful. Atthe same time, the combustible material in the tailings is burned up andthe water passes off as vapor, so the en tire composition of thetailings is disposed of. Operation of the apparatus for carrying out theprocess is relatively inexpensive, especially since after the initialstart-up by the burner the rest of the burning is autogenous with thetailings supplying their own fuel.

According to the provisions of the patent statutes, we have explainedthe principle of our invention and have illustrated and described whatwe now consider to represent its best embodiment. However, we desire tohave it understood that, within the scope of the appended claims, theinvention may be practiced otherwise than as specifically illustratedand described.

We claim:

1. The non-polluting method of disposing of the ash that forms part of aslurry of water and particles of combustible material and ashconstituting the tailings from a coal preparation plant, comprisingcontinuously delivering the slurry to a combustion zone, maintaining afluidized bed of said particles in said zone, establishingself-sustaining combustion of the combustible material in said bed andmaintaining it at a temperature that causes the ash particles therein toagglomerate into inert water-insoluble pellets, and removing saidpellets from said bed at a rate that maintains the desired pressure dropacross the bed.

2. The non-polluting method recited in claim 1, including maintainingsaid fluidized bed by continuously forcing air up through it, preheatingsaid air, and controlling said preheating to control said temperature.

3. The non-polluting method recited in claim 1, including controllingsaid temperature by controlling the rate of said delivery of slurry tosaid combustion zone.

4. The non-polluting method recited in claim 1, including controllingsaid temperature to confine it to a range between about l,400 and l,700F.

5. The non-polluting method recited in claim 1, including firstintroducing a fluidized bed of inert granular material into saidcombustion zone, heating that bed to a temperature high enough to ignitesaid combustible material, then starting delivery of said slurry to thecombustion zone, and stopping said heating when said combustiblematerial in the bed ignites and supports combustion in the bed.

6. The non-polluting method recited in claim 1, including maintainingsaid slurry with a solids content between about 35 percent and 45percent by weight.

7. The non-polluting method recited in claim 1, including collectingsaid tailings in a collector for said delivery to said combustion zone,and maintaining the desired density of the slurry in the collector byreturning part of the slurry thereto before it reaches said combustionzone.

8. The non-polluting method recited in claim 1, including collectingdust leaving said fluidized bed, and returning the dust to saidcombustion zone.

9. The non-polluting method recited in claim 1, including adding to saidslurry as it is being delivered to the combustion zone a material thatlowers the fusion point of said ash.

1. The non-polluting method of disposing of the ash that forms part of aslurry of water and particles of combustible material and ashconstituting the tailings from a coal preparation plant, comprisingcontinuously delivering the slurry to a combustion zone, maintaining afluidized bed of said particles in said zone, establishingself-sustaining combustion of the combustible material in said bed andmaintaining it at a temperature that causes the ash particles therein toagglomerate into inert waterinsoluble pellets, and removing said pelletsfrom said bed at a rate that maintains the desired pressure drop acrossthe bed.
 2. The non-polluting method reciteD in claim 1, includingmaintaining said fluidized bed by continuously forcing air up throughit, preheating said air, and controlling said preheating to control saidtemperature.
 3. The non-polluting method recited in claim 1, includingcontrolling said temperature by controlling the rate of said delivery ofslurry to said combustion zone.
 4. The non-polluting method recited inclaim 1, including controlling said temperature to confine it to a rangebetween about 1,400* and 1,700* F.
 5. The non-polluting method recitedin claim 1, including first introducing a fluidized bed of inertgranular material into said combustion zone, heating that bed to atemperature high enough to ignite said combustible material, thenstarting delivery of said slurry to the combustion zone, and stoppingsaid heating when said combustible material in the bed ignites andsupports combustion in the bed.
 6. The non-polluting method recited inclaim 1, including maintaining said slurry with a solids content betweenabout 35 percent and 45 percent by weight.
 7. The non-polluting methodrecited in claim 1, including collecting said tailings in a collectorfor said delivery to said combustion zone, and maintaining the desireddensity of the slurry in the collector by returning part of the slurrythereto before it reaches said combustion zone.
 8. The non-pollutingmethod recited in claim 1, including collecting dust leaving saidfluidized bed, and returning the dust to said combustion zone.
 9. Thenon-polluting method recited in claim 1, including adding to said slurryas it is being delivered to the combustion zone a material that lowersthe fusion point of said ash.